1. Field of the Invention
The present invention relates to an electronic apparatus provided with a cooling unit that cools circuit components, such as a microprocessor, which generate heat during operation, and more specifically, to the structure of a cooling unit having a circulating path through which a liquid coolant flows and an air-cooled heat sink.
2. Description of the Related Art
A microprocessor is incorporated in, for example, notebook-type portable computers. The heat that the microprocessor generates while operating is increasing as its data-processing speed rises, and it performs more and more functions. The higher the temperature of the microprocessor, the less efficiently it operates. To cool the microprocessor, so-called cooling systems of the liquid-cooling type have been developed in recent years. A liquid-cooling system uses a liquid coolant that has a far higher specific heat than air.
Japanese Patent Application KOKAI Publication No. 7-142886 discloses a cooling system of the liquid-cooling type, configured for use in a portable computer. The cooling system comprises a heat-receiving header, a heat-radiating header, and a tube for circulating the liquid coolant. The heat-receiving header is accommodated in a housing of the portable computer and thermally connected to the microprocessor. The heat-radiating header is accommodated in a display unit supported by the housing. The tube is arranged to extend between the housing and the display unit to connect the heat-receiving header and the heat-radiating header together.
With this cooling system, the liquid coolant absorbs heat from the microprocessor in the heat-receiving header. The liquid coolant thus heated is transferred to the heat-radiating header through the tube. While passing through the heat-radiating header, the liquid coolant releases heat from the microprocessor. The liquid coolant cooled by the heat-radiating header returns to the heat-receiving header through the tube. Then, the liquid coolant absorbs heat from the microprocessor again. This circulation of the liquid coolant efficiently transfers heat from the microprocessor to the heat-radiating header. This serves to improve cooling performance for the microprocessor compared to common conventional cooling systems of the air-cooling type.
The heat-radiating header used in the cooling system of the liquid-cooling type has a coolant channel through which the liquid coolant flows. It is also thermally connected to a metal housing of the display unit. With this arrangement, heat from the microprocessor absorbed by the liquid coolant is diffused by heat conductance from the heat-radiating header to the metal housing. The heat is then released to the air from the surface of the metal housing. In other words, an increase in the value of heat conducted from the heat-radiating header to the metal housing, increases the surface temperature of the metal housing and enhances the radiating performance of the heat-radiating header.
An operator touches the metal housing of the display unit when carrying the potable computer or closing and opening the display unit. Thus, the surface temperature of the metal housing must not be thoughtlessly increased. Considering thermal effects on the operator, the surface temperature of the metal housing must be kept equal to or lower than, for example, 60° C.
However, under this thermal condition, the allowable amount of heat released from the heat-radiating header is estimated to be at most between 10 and 20 W. The microprocessor for the portable computer is expected to have its performance further improved in the future. The value of heat generated by the microprocessor is expected to correspondingly increase sharply. Accordingly, the current cooling system of the liquid-cooling type cannot cope with an increase in the amount of heat generated by the microprocessor. Therefore, the cooling performance for the microprocessor may be insufficient.